Dispersion hardened metals having improved oxidation characteristics at elevated temperature

ABSTRACT

Dispersion hardened metals based on nickel and containing from 8-35 percent of chromium (refractory oxides, such as thoria, are pervasively dispersed in the metal to harden the same) are modified by the inclusion of from about 2.5 to about 5 percent of aluminum in order to provide improved oxidation resistance at elevated temperature while subjected to high velocity gases.

United States Patent [1 1 Baranow et a1.

DISPERSION HARDENED METALS HAVING IMPROVED OXIDATION CHARACTERISTICS AT ELEVATED TEMPERATURE Inventors: Sanford Baranow; Leo J. Klingler, both of Baltimore, Md.

Assignee: Cabot Corporation, Boston, Mass.

Filed: May 6, 1971 Appl. Neil $0,952

US. Cl. 148/32, 29/1825, 75/.5 AC,

75/171, 75/206, 148/11.5 F Int. Cl. C22c 19/00 Field of Search 75/.5 AC, .5 BC,

Primary Examiner-Richard 0. Dean A ttorney- Kenneth W. Brown and Eugene F. Buell [5 7] ABSTRACT Dispersion hardened metals based on nickel and containing from 8 -35 percent of chromium (refractory oxides, such as thoria, are pervasively dispersed in the metal to harden the same) are modified by the inclusion of from about 2.5 to about 5 percent of aluminum in order to provide improved oxidation resistance at elevated temperature while subjected to high velocity gases.

9 Claims, N0 Drawings DISPERSION HARDENED METALS HAVING IMPROVED OXIDATION CHARACTERISTICS AT ELEVATED TEMPERATURE The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

The present invention relates to dispersion hardened metals based on nickel, and containing chromium, which are modified to improve high temperature erosion resistance by the inclusion of from about 2.5 to about percent of aluminum. The term erosion is used loosely to identify oxidation behavior at elevated temperature under the influence of high velocity gases. As will be appreciated, repeated re-entry of space vehicles requires that metal surfaces be capable of withstanding prolonged subjection to high velocity gases while the metal surfaces are quite hot. Similar oxidative erosion stresses will occur within high performance jet engines.

The invention proposes metal structures possessing improved capacity to resist the high temperature oxidative erosive stresses which have been referred to.

The invention is directed to metal structures which are prepared by powder metallurgy procedures in which the powder is consolidated under pressure, sintered, densified by extruding, forging, or rolling, and then formed as desired and then recrystallized, though recrystallization may occur as part of the desired forming at elevated temperature. The production of formable sheet is particularly contemplated, but the alloys can also be formed into final form by extrusion.

More particularly, the powder which is consolidated comprises nickel, from 8 to 35 percent chromium, from about 2.5 to about 5 percent of aluminum, and a minor proportion of finely divided particles of a refractory metal oxide having a free energy of formation at l,O00 C. greater than 100 kilocalories per gram atom of oxygen which is incorporated in the powder. These refractory metal oxides are illustrated herein by thoria, but the class of useful oxides is well known.

The present invention is an improvement falling within the generic disclosure of Lambert and Marsh U.S. Pat. Nos. 3,479,180 and 3,556,769. Thus, the process of preparing the alloys under consideration is described and claimed in said Patent 3,479,180, and the blended powder mixtures used in the process are claimed in Patent 3,556,769.

The aluminum component and its proportion are critical to this invention. In the chromium-containing dispersion modified nickel alloys under consideration, it has been found that the compound Cr O is present at the surface of the alloy. On exposure to the high temperature oxidative conditions under consideration, CrO is formed and, particularly in the presence of the high velocity gas impact which, on re-entry takes place under a vacuum of about Torr, volatilizes which is detrimental to the integrity of the exposed piece. When the nickel-chromium dispersion modified alloys of this invention contain above 2 .5 percent of aluminum, then it would appear that the surface of the formed alloy becomes covered with a mixture of A1 0 and NiAl- O,. These compounds provide a gray colored surface which resists volatilization under the high temperature oxidative conditions referred to hereinbefore. This gray dative erosion. Nonetheless, it is usually desired to employ at least 3 percent of aluminum to insure that the gray Al O -NiAl Q, composition will uniformly cover the entire surface of the alloy product.

It is desired in this invention to form pieces, and especially sheet, which can be extensively formed or fabricated with confidence that the formed or fabricated final structure will resist breakage resulting from embrittlement of the product. At about 4.5 percent alumi num, there begins to be formed traces of a nickelaluminum gamma phase intermetallic compound which has an embrittling tendency. The presence of the gamma material becomes particularly noticeable above 5 percent of aluminum.

The preferred proportion of chromium, particularly when the production of formable sheet is intended, is from 10-30 percent, and the proportion of thoria or other refractory oxide is commonly 0.5-6 percent, preferably from 1-4 percent. These oxide particles normally have an average particle size less than about 200 millimicrons.

The alloys of this invention may be entirely constituted by nickel, chromium, aluminum and dispersed oxide, but some minor additions may be tolerated or desired. Thus, from 0-l.0 percent, preferably from 0-0.5 percent of the alloy may be constituted by yttrium. Instead of yttrium, one may use lanthanum, hafnium, or rare earths of atomic numbers 58-71. From the standpoint of materials which may be present in somewhat larger amount, iron can be tolerated in an amount up to about 15 percent, but significant inclusions other than iron are not desirable. The preferred products of this invention are single phase alloys in which any miscellaneous additives which may be included are insufficient to destroy the preferred single phase alloy matrix which is desired.

The invention is illustrated in the examples which follow.

EXAMPLE 1 This example describes the preparation of a Ni-16Cr- 3.5Al alloy powder containing approximately 2 volume percent Th0: by a milling process. One part by weight of a powder alloy of composition Cr-l8Al(l8% Albalance Cr) and 4 parts of a Ni-2.7 Th0: powder were blended in a twin-shell cone blender. The Ni-2.7 Th0 powder had been prepared by a process involving coprecipitation followed by hydrogen reduction, as described in Example 4 of U.S. Pat. No. 3,085,876. The Cr-lSAl powder used had an average particle size, as measured by a Fisher Sub-Sieve Sizer, of 4.5 microns. 96 pounds of this blended powder slurried with 3 gallons of pure benzene were poured into a single-shaft, 15 gallon attritor mill which had been charged with as inch diameter steel balls. The powder was attrited for 1% hours at 240 RPM, discharged through a screen to separate the balls from the slurry and the latter was air dried.

EXAMPLE 2 10 pounds of the Ni-16Cr-3.5Al.-2.2Th0 powder produced in Example 1 were hydrostatically compacted to 70 percent of theoretical density to form a rectangular compact which was canned in a mild steel container. The canned compact was then purged with hydrogen and heated slowly to 2,100 F. The compact was then cooled, evacuated and welded shut. The compact was then forged to 100 percent density at 1,750F. to form a 5.5 X 3.5 X 0.615 inch slab. The slab was then reheated and rolled at 2,200 F. to a thickness of 0.095 inch and recanned in stainless steel. Final rolling was then carried out at 2,200F. to provide a sheet having a thickness of 0.02 inch. The steel can was removed by machining and the sheet was trimmed to remove edge cracks. This sheet was then heat treated in air for 1 hour at 2,400F. and then surface ground to remove scale and thereby provide a sheet having a thickness of 0.015 inch. Final chemical analysis showed a chromium content of 15.87 percent, an aluminum content of 3.49 percent, and a thoria content of 2.28 percent. The balance of the composition was substantially entirely nickel, with impurities being negligible.

The sheet product of the invention possesses exceptional oxidation resistance under highly erosive circumstances, e.g., by being repeatedly subjected to 1% hour cycles at 2,200F. in a high velocity gas stream. The gas stream was provided by burning liquified propane and directing the combustion products against the surface of the sheet. After 100 cycles, or 50 hours of testing, it was found that less than 0.1 mil of metal had been lost and that only a slight trace of internal oxidation had resulted.

Not only is the sheet product strongly resistant to oxidation at elevated temperature in the presence of high velocity gases, but the product exhibits excellent strength and ductility characteristics as indicated below.

Room Temperature 2,000F.

Transverse UTS (KS1) 162.6 13.0 Transverse YS (KS1) 122.4 11.6 Elongation (71) 11.5 5.5

Moreover, the product exhibits a single phase alloy matrix which enables the sheet to be extensively fabricated without inducing brittleness.

While the invention has been particularly illustrated by the production of a sheet product, it will be understood that the same alloy can be formed to desired shape by extrusion.

The invention is defined in the claims which follow, all proportions referred to herein and in the claims being by weight.

We claim:

1. A dispersion hardened metal having a density of substantially percent of the theoretical density and characterized by improved resistance to oxidation at elevated temperature while subjected to high velocity gases and consisting essentially of nickel, from 8 to 35 percent of chromium, and from about 2.5 to about 5 percent of aluminum, said metal having pervasively dispersed therein a minor proportion of finely divided particles of a refractory metal oxide having a free energy of formation at 1,000 C. greater than about 100 kilocalories per gram atom of oxygen.

2. A dispersion hardened metal as recited in claim 1 in which said refractory metal oxide is dispersed in an amount of from 0.1 to 6 percent by volume and has an average particle size less than about 200 millimicrons.

3. A dispersion hardened metal as recited in claim 1 in which said refractory metal oxide is thoria.

4. A dispersion hardened metal as recited in claim 1 in which said aluminum is present in an amount of at least 3 percent.

5. A dispersion hardened metal as recited in claim 4 in which said chromium is present in an amount of from 10-30 percent.

6. A dispersion hardened metal as recited in claim 1 in which said metal is in the form of a sheet.

7. A dispersion hardened metal as recited in claim 1 in which said metal has a single phase alloy matrix.

8. A dispersion hardened metal as recited in claim 1 in which said metal includes from 0 1.0 percent of yttrium, lanthanum, hafnium or rare earths of atomic numbers 58-71.

9. A dispersion hardened metal as recited in claim 1 in which said metal includes up to 15 percent of iron. 

2. A dispersion hardened metal as recited in claim 1 in which said refractory metal oxide is dispersed in an amount of from 0.1 to 6 percent by volume and has an average particle size less than about 200 millimicrons.
 3. A dispersion hardened metal as recited in claim 1 in which said refractory metal oxide is thoria.
 4. A dispersion hardened metal as recited in claim 1 in which said aluminum is present in an amount of at least 3 percent.
 5. A dispersion hardened metal as recited in claim 4 in which said chromium is present in an amount of from 10-30 percent.
 6. A dispersion hardened metal as recited in claim 1 in which said metal is in the form of a sheet.
 7. A dispersion hardened metal as recited in claim 1 in which said metal has a single phase alloy matrix.
 8. A dispersion hardened metal as recited in claim 1 in which said metal includes from 0 - 1.0 percent of yttrium, lanthanum, hafnium or rare earths of atomic numbers 58-71.
 9. A dispersion hardened metal as recited in claim 1 in which said metal includes up to 15 percent of iron. 